1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to a LCD device having thin film transistors (TFTs) and to a method of manufacturing the same.
2. Description of Related Art
In general, a liquid crystal display (LCD) device displays an image using a plurality of pixels. An LCD device that uses thin film transistors (TFTs) as switching elements is typically called a thin film transistor liquid crystal display (TFT-LCD) device.
A liquid crystal display device uses the optical anisotropy and polarization properties of liquid crystal molecules. Because of their peculiar characteristics liquid crystal molecules have a definite orientational order in arrangement. The arrangement direction of liquid crystal molecules can be controlled by an applied electric field. In other words, when electric fields are applied to liquid crystal molecules, the arrangement of the liquid crystal molecules changes. Since incident light is refracted according to the arrangement of the liquid crystal molecules, due to the optical anisotropy of liquid crystal molecules, image data can be displayed.
An active matrix LCD (AM-LCD) has its thin film transistors (TFTs) and pixel electrodes arranged in a matrix. Such LCDs can have high resolution and superior imaging of moving images.
FIG. 1 is a cross-sectional view illustrating a conventional liquid crystal display (LCD) panel. As shown in FIG. 1, the LCD panel 20 has lower and upper substrates 2 and 4 with a liquid crystal layer 10 interposed therebetween. The lower substrate 2, which is referred to as an array substrate, has a TFT. “S” as a switching element that changes the orientation of the liquid crystal molecules. A pixel electrode 14 applies a voltage to the liquid crystal layer 10 according to the state of the TFT “S”. The upper substrate 4 has a color filter 8 for implementing a color and a common electrode 12 on the color filter 8. The common electrode 12 serves as an electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 14 is arranged over a pixel portion “P”, of a display area. Further, to prevent leakage of the liquid crystal layer 10, the two substrates 2 and 4 are sealed using a sealant 6.
FIG. 2 is a plan view illustrating an array substrate. A gate line 22 is arranged in a transverse direction and a data line 24 is arranged in perpendicular to the gate line 22. A pixel region having a pixel electrode 14 is defined by the gate line 22 and the data line 24.
In an AM-LCD, the switching element (TFT “S”) that selectively applies the voltage to the liquid crystal layer 10 (see FIG. 1) is formed near the crossing of the gate line 22 and the data line 24. The TFT “S” has a gate electrode 26 that is extended from the gate line 22, a source electrode 28 that is extended from the data line 24, and a drain electrode 30 that is electrically connected to the pixel electrode 14 via a contact hole 31.
A gate pad 21 is formed at one end of the gate line 22, and a data pad 23 is formed at one end of the data line 24. The gate and data pads 21 and 23 are electrically connected with external drive circuitry (not shown) that operates the TFT “S” and thus the pixel electrode 14. The gate line 22 and the pixel electrode 14 form a storage capacitor “Cst” which stores electric charges.
When the gate line 22 receives gate signals, the TFT “S” turns ON. The information on the data line 24 is then applied to the pixel electrode 14. The applied electric field from the pixel electrode 14 then changes the arrangement direction of the liquid crystal molecules, causing the liquid crystal molecules to refract the light generated by a back light device. When the gate line 22 turns the TFT “S” to the OFF-state, data signals are not transmitted to the pixel electrode 14. In this case, the arrangement of the liquid crystal is not changed, and thus the direction of the light from back light device is not changed.
When fabricating a liquid crystal panel, a number of complicated process steps are required. In particular, the TFT array substrate requires numerous mask processes. Each mask process requires a photolithography process. Thus, to reduce cost and manufacturing time, the number of mask processes should be minimized.
In general, a manufacturing process depends on the materials used and on the design goals. For example, the resistivity of the material used for the gate lines and the data lines impacts the picture quality of large LCD panels (over 12 inches) and of LCD panels having high resolution. With such LCD panels, a material such as Aluminum (Al) or Al-alloy is often used for the gate lines and the data lines.
In LCD devices having a high aperture ratio, a method of back exposure is employed when forming the pixel electrode 14. That method will now be explained.
FIGS. 3A to 3E are cross-sectional views taken along line III-III and illustrate the process steps of fabricating a conventional TFT array substrate for an active matrix LCD device.
An inverted staggered type TFT is generally used due to its simple structure and superior efficiency. The inverted staggered type TFT can be classified as either a back channel etched type (EB) and an etch stopper type (ES), depending on the fabrication method that is used. The fabrication method of the back channel etched type TFT will now be explained.
A first metal layer is deposited on a substrate 1 by a sputtering process. The substrate previously underwent a cleaning process to enhance adhesion between the substrate 1 and the first metal layer. That cleaning process removes organic materials and alien substances from the substrate.
FIG. 3A shows a step of forming a gate electrode 26 by patterning the first metal layer. The gate electrode 26 is usually Aluminum, which reduces the RC delay owing to a low resistance. However, pure Aluminum may result in line defects caused by formation of hillocks during a subsequent high temperature process. Thus, an Aluminum alloy or another material is beneficially used.
Referring to FIG. 3B, a gate insulation layer 50 is formed over the surface of the substrate 1 and over the gate electrode 26. Then, a pure amorphous silicon (a-Si:H) layer and a doped amorphous silicon (n+ a-Si:H) layer are formed in sequence on the gate insulation layer 50, and then patterned to form an active layer 52 and an ohmic contact layer 54. The ohmic contact layer 54 reduces the contact resistance between the active layer 52 and electrodes that will be formed later.
As depicted in FIG. 3C, source and drain electrodes 28 and 30 are formed by depositing and patterning a second metal layer. A portion 51 of the ohmic contact layer 54 on the active layer 52 is etched using the source and drain electrodes 28 and 30 as masks. If the ohmic contact layer 54 between the source and drain electrodes 28 and 30 is not removed, serious problems of deteriorated electrical characteristics and low efficiencies of the TFT “S” (see FIG. 2) can result. Etching the portion of the ohmic contact layer 54 over the gate electrode 26 requires special attention. While etching the ohmic contact layer 54, the active layer 52 is typically over-etched by about 50˜100 Å due to the fact that the active layer 52 and the ohmic contact layer 54 have no etch selectivity.
As shown in FIG. 3D, an insulating protection layer 56 is formed over the substrate 1 and over the source and drain electrodes 28 and 30. The protection layer protects the active layer 52. The protection layer is etched to form a drain contact hole 31 that is used to connect the drain electrode 30 to a pixel electrode 14 that is formed later.
Due to the unstable energy state of the active layer 52, and to residual substances that are generated during etching that can affect the electrical characteristics of the TFT, the protection layer 56 is usually made of an inorganic material, such as SiNx and SiO2, or an organic material such as a BCB (benzocyclobutene). In addition, the protection layer 56 should have a high light transmittance, a high humidity resistance, and a high durability in order to protect the channel area and major portions of a pixel region from humidity damage and scratches that can occur during later process steps.
FIG. 3D also shows a step of forming a pixel electrode 14 by depositing a Transparent Conducting Oxide (TCO) layer and by forming a photoresist “PR”. Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO) is usually used for the Transparent Conducting Oxide (TCO) layer. The photoresist “PR” is a material which, when being subject to light irradiation through a mask, absorbs light energy to cause a photochemical reaction and to form a latent image. To obtain a high aperture ratio, a negative photoresist is preferably used. The portion of the photoresist that does not absorb light is removed during a developing process.
When using a negative photoresist, a back exposure is used in the patterning process. Namely, in the conventional method of fabricating an LCD device, the pixel electrode is formed using a negative photoresist in order to enhance the aperture ratio. With the negative photoresist in place, a back exposure and a front exposure are performed simultaneously to form the pixel electrode. As shown in FIG. 3D, areas “B” of the photoresist “PR” are exposed by the back exposure and area “F” of the photoresist “PR” is exposed by the front exposure. In the front exposure process, a mask 58 is required for the area “F” (which is not exposed by the back exposure) to make the pixel electrode 14 have a contact with the drain electrode 30.
Referring to FIG. 3E, a high aperture ratio is produced by the back exposure due to the fact that the area of the pixel electrode 14 is enlarged.
FIG. 4 is a cross-sectional view taken line IV-IV of FIG. 2 and illustrates gate and data pads. A gate pad 21 is initially formed on the substrate 1. Then, the gate insulation layer 50 is deposited and etched to produce a gate pad contact hole that exposes a portion of the gate pad 21. A data pad 23 is then formed on the gate insulation layer 50. Then, the protection layer 56, which is patterned to have gate and data pad contact holes that expose the gate and data pads, is formed. Gate and data pad electrodes 62 and 60, which electrically connect to the gate and data pads 21 and 23, via the corresponding contact holes, are then formed on the protection layer 56.
FIG. 5 is a flow chart illustrating the manufacturing process steps of the LCD device as shown in FIGS. 3A to 3E.
In the first step, ST200, a glass substrate is cleaned by a cleaning process. That cleaning process enhances adhesion between the substrate and the first metal layer by removing organic materials, alien substances, and particles from the substrate.
In the second step, ST210, the first metal layer, which may be of Aluminum or Molybdenum, is deposited. Then, the gate electrode and a first capacitor electrode, which are portions of the gate line, are formed by lithography.
In the third step, ST220, the gate insulation layer and the semiconductor layers (the active layer and the ohmic contact layer) are sequentially formed. The gate insulation layer is beneficially comprised of Silicon Oxide or of Silicon Nitride and has a thickness of about 3000 Å.
In the fourth step, ST230, the source and drain electrodes are formed by depositing and patterning a metallic material such as Chrome (Cr) or Cr-alloy.
In the fifth step, ST240, a channel region is formed by removing a portion of the doped amorphous silicon layer (ohmic contact layer) between the source and drain electrodes. In this step, the source and drain electrodes are used as masks.
In the sixth step, ST 250, the protection layer is formed to protect the other elements. The protection layer includes contact holes and is made of a material having a high light transmittance, a high humidity resistance, and a high durability.
In the seventh step, ST260, a transparent conductive electrode, beneficially comprised of ITO (Indium-Tin-Oxide) is-formed. Then, the pixel electrode is formed by using back and front exposures as previously described. The gate and data pad electrodes are also formed in this step.
As described above, the prior art requires various masks when fabricating the TFT array substrate of an LCD device, and each mask process requires several steps such as a cleaning step, a depositing step, a baking step and an etching step. Therefore, if the number of mask processes is decreased by only one mask, the throughput and manufacturing yields can dramatically increase and the manufacturing costs and time can be reduced. Furthermore, in the conventional art process of forming pixel electrodes, although a back exposure is used an additional mask for the front exposure is necessary.